This is the first time I have had a character in position to do the MallyX quest. I assume, that if the quest is repeatable, the rewards are the same each time. Is this correct? If so, I am about to embark on getting an Ambrace. If not, I guess no Tormented weapon in HoM, seeings how I only have 85k in gold.

Yes, the rewards are the same each time you do any quest. And I'm pretty sure that applies to all quests in GW. The only "exception" is that you only get 1 end-game reward trophy (like Droknar's Key, Book of Secrets etc.) on your first time completing the last mission of a campaign.

Just in case you're curious, I'd like to share some other information just in case.
The 4 main DoA areas (Torc'qua, Stygian, Ravenheart, Foundry) are all connected and all reward you with one chest after killing a boss (so 4 chests in that entire explorable area). The first chest give you 1 gemstone (2 in HM), the next gives 2 gems (4 in HM), then 3 gems (6 in HM), and the last gives 4 gems (8 in HM). Each gemstone type is given by its respective area so:
Torc'qua -> Margonite Gems
Stygian -> Stygian Gems
Ravenheart -> Torment Gems
Foundry -> Titan Gems
So it's quite profitable to try to do all 4 of them in one session. Just make sure to have a Mercantile Summoning Stone because a large amount of enemies will give a large amount of drops.

Yes. Once you've killed Mallyx and collected your reward, you can pick up the quest again from the NPC in GoA outpost and go through the same routine. Rewards will remain the same (iirc 1 of each gem).